The mounted structure is manufactured by following a series of SMT processes.
At first, solder paste which is constituted by spherical solid metal solder and liquid flux is applied with mask print on a substrate electrode which is provided on a surface of a substrate. Then, an electronic part such as a tip condenser or an IC having an electronic part electrode is mounted on the solder paste. And solder contained in the solder paste is melted by heating up to a temperature that is higher than the melting point of the solder by 20° C. to 30° C. At the end, the solder is solidified by cooling, and electrical connection of the substrate electrode and the electronic part electrode is carried out.
Since the solder is heated in the case of such an SMT process, the electronic components and the substrate are heated to the temperature that is higher than the melting point of the solder by 20° C. to 30° C. Therefore, by using the Sn-58Bi solder with the melting point of 138° C., electric components which have low heat resistance can be mounted on the substrate by low-temperature heating at 160° C. or less with SMT.
However a metal structure of the Sn-58Bi solder is a eutectic structure of Sn and Bi, and Bi is poor in ductility. Therefore a crack occurs on an interface of Sn and Bi when a shock or vibration is added.
In addition, a good heat-resistant fatigue characteristic is not acquired, because metal crystals of the Sn-58Bi solder alloy become a fault size easily by heat and the strength of a solder bonding part decreases. Therefore a crack due to repetition of the heat fatigue is easy to occur. Because an electrical problem and a mechanical problem occur when a crack occurs and progresses, it is necessary to restrain an occurrence of the crack.
As a conventional mounted structure, a mounted structure in which junction reliability is improved by using Sn-58Bi solder and by reinforcing with resin or the like is disclosed (for example, refer to Japanese Patent Laid-Open No. 2006-186011).
FIG. 17 is a figure showing a conventional mounted structure mentioned in Japanese Patent Laid-Open No. 2006-186011.
The conventional mounted structure has a solder bonding part 205 using Sn-58Bi solder, which bonds between a component electrode 204 of an electric component 203 and a substrate electrode 201 of a substrate 200, a first adhesion reinforcing part 206 using a thermosetting resin, which is formed outside of the electric component 203 and the solder bonding part 205, a solder part 208 using Sn-58Bi solder, a second adhesion reinforcing part 207 using a thermosetting resin of which an essential agent is an epoxy-system materials, and resist 202.